Long-Range Effects of Acyl Groups in the Solvolysis of Glycofuranosyl Halides. The Synthesis of 2,3-Di-O-benzyl-5-O-p-nitrobenzoyl-α-D- arabinofuranosyl Chloride and of 2-O-Benzyl- 3,5-di-O-p-nitrobenzoyl-α-D-arabinofuranosyl Chloride